Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	$ 4,586,151	$ 3,852,664
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,274,116	3,450,252
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	312,035	402,412
U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	112,805	64,982
Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	692,988	587,677
Foreign Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	181,310	196,750
Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	421,337	197,874
Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,077,417	1,812,162
Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	788,259	574,269
Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,645	15,525
Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	296,390	386,887
Non-Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value		16,538
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	312,035	402,412
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	312,035	402,412
Level 1 | Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,645	15,525
Level 1 | Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	296,390	386,887
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	4,265,758	3,433,714
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,265,758	3,433,714
Level 2 | U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	112,805	64,982
Level 2 | Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	692,988	587,677
Level 2 | Foreign Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	181,310	196,750
Level 2 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	421,337	197,874
Level 2 | Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,077,417	1,812,162
Level 2 | Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	779,901	574,269
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	8,358	16,538
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	8,358	16,538
Level 3 | Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	$ 8,358
Level 3 | Non-Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value		$ 16,538